This invention relates to digital printing and more particularly to printing using devices which eject ink onto the printed substrate. However, the invention is not limited to ink ejection devices and is also applicable to laser, light emitting diode printers and to digital photocopiers.
In ink ejection devices a printhead has an array of nozzles through which ink is selectively ejected onto the substrate as the substrate moves relative to the printhead. The printhead may print by scanning across the substrate to print horizontal bands or, if it is a full page width printhead, it may pass along the length of the page. A blocked nozzle will result in multiple horizontal blank lines, in the case of a scanning type printhead, or a blank vertical line in the case of a page width printhead. Such blank lines are undesirable since they detract from the printed result.
The present invention provides a method of modifying the printing of an image so as to reduce or effectively eliminate the visual effect of one or more such blocked nozzles apparent to the eye of an observer in normal use. However, the invention is applicable to other forms of printing where a device, whether passive or active, is repeatedly used to produce dots of ink or the like on a substrate. The invention has potential application to laser and LED type printers and photocopiers where a fault in the imaging drum or light source can result in repeated faults in the image produced. As used above and throughout the description and claims the term image is to be understood to have a broad meaning and includes anything printed, such as text and line drawings.
In one broad form the invention provides a method of modifying an image to be digitally printed by a printing device to compensate for failure to correctly print dots of ink at specific locations, the method including the steps of:
a) identifying said specific location or locations, and
b) adding at least one additional dot at at least one additional location adjacent or near to the respective specific location compared to that required by the image data.
In another broad form the invention also provides a printer having a row of activatable devices which, when activated, cause rows of dots to be deposited onto a substrate and means to move the substrate relative to the row of devices in a direction generally perpendicular to the row of dots, said printer including:
means to determine if one or more of said devices is not operating correctly; and
control means for analysing images or image data and for identifying a specific location or locations where a dot of ink should be printed by activation of a incorrectly operating device and one or both of the devices on either side of the failed device to produce extra dots of ink compared to that required by the image data.
Extra ink dots may be merely located to the side of the respective specific location if the adjacent location is unused by the image. One or more extra ink dots may be placed to the side and above or below the respective specific location or both above and below the respective specific location. Two or more extra ink dots may be provided in each quadrant relative to the respective specific location.
The extra ink dots may be the same size as those normally required by the image data or may be larger or smaller. Ink dots required by the image data adjacent to where extra dots are printed may be reduced in size to accommodate the extra ink dots. Depending on xe2x80x9cnormalxe2x80x9d ink dot size and spacing and the number and size of extra dots and any change in size of xe2x80x9cnormalxe2x80x9d dots adjacent to the extra dots, the extra dots may overlap with themselves or xe2x80x9cnormalxe2x80x9d dots or both or may be distinct non overlapping dots The extra dots are preferably printed on both transverse sides of the specific locations.